The invention concerns a radial piston pump.
Radial piston pumps are, among other types of pumps, employed in motor vehicles, for lubrication medium pumping in internal combustion engines and transmissions. Further applications are found in pumps for hydraulic positioning members, steering joints, springs, couplings, stepless drives, automatic controlled transmissions, hydraulic driving aids, auxiliary drives, concrete mixers, and the like. These pumps characterize themselves as particularly adaptable in for installations wherein high pressures are involved.
Classified as positive displacement pumps, these transport the pressure medium, not in continuous flow, but non-uniformly in increment volumes per rotation of an eccentrically centered drive cam. The cyclically transported volumes bring about, both on the suction side as well as on the pressure side, variations in pressure, pressure oscillations, or pulsations. These pressure variances are superimposed on intake and output impulses, which, arise by the opening and closing of the transport chamber. The pulsations are especially great, if during the use of inlet or outlet valves of the coil spring type, application volumes with large pressure variations are suddenly connected. Further, intense pressure swings may occur, if the pressure in the system is high or the cylinder is only partially filled.
When the pressure in a cylinder attains such a pressure as to trigger the opening of the annular coil spring valve closure element, then this element rises from its seating in the area of the respective cylinder and the hydraulic medium, i.e. the hydraulic fluid, is forced into the plenum. If the pressure in the cylinder falls under the closure point of the coil spring valve, then the respective closure element is forced back onto said seat and thereby causes a loud hammer sound. This procedure repeats itself for each rotation of the drive cam, and as often as the number of the piston-cylinder units in the pump.
The said noise is just so much louder, the more dynamically the opening and closing process runs, that is, in accord with how great the ratio is between the pressure at opening and closing and further, in accord with the slope of the pressure increase curve from the instant of the opening. If these several values run very high, then the coil spring valve element is lifted distant from its seat very quickly and subsequently strikes accordingly hard on said seat upon its return.
DE 43 38 641 A1 makes known a radial piston pump, in which a coil spring valve lies on a seat located on the circumference of a pump body and which covers the pressure opening of the cylinder. In order to keep the threshold of the opening pressure of the said valve coil spring to a minimum, the exit sides of the pressure openings are provided with stress relieving corrugations, which run in the circumferential direction of the seating surfaces. By this means, the surfaces of the coil spring valve initially loaded by the opening pressure are increased, so that the necessary opening pressure is lessened. Further, the opening thrust remains small, since the stress relieving corrugations apportion the hydraulic fluid over nearly the entire length of the circumference, so that the excess pressure increase remains small. As a result of the small opening thrust of the coil spring valve, noise is also reduced.
DE 43 37 144 A1 discloses a radial piston pump, in which elastic rubber ring segments are placed, which are pressed against sealing surfaces proximal to the pressure opening. These surfaces, on which said segments seat, are provided with concentric corrugations, which communicate with the interior of the cylinder by means of small borings. Since the underside of the sealing segments, at the beginning of the transport thrust are not only stressed over the projected area of the pressure opening, but additionally over the projected area of the corrugations, the surface of the sealing segments is enlarged, thus acting pressure of the given opening may be kept small.
Thus, the invention has the purpose of reducing the noise level of generic type radial piston pumps.
In accordance with the invention, a stress relieving zone of a piston-cylinder unit is connected by means of a conduit to the pressure zone of a neighboring piston-cylinder unit. This arrangement achieves the goal, that the pressure between two neighboring piston-cylinder units can be equalized, so that pressure peaks are ameliorated. Further, the pressure in a piston-cylinder unit already in transport thrust supports the opening pressure in the neighboring piston-cylinder lagging in its transport thrust. By this means, the coil spring valve in the area of a pressure opening is peremptorily controlled by the higher pressure of the neighboring, preceding piston-cylinder unit. The result of this is that the coil spring valve is lifted with a lesser thrust, over a relatively greater circumferential zone, between two coacting piston-cylinders.
The conduit between the relief zone and the pressure area of a neighboring piston-cylinder unit can be constructed in a very simple manner in a radial piston pump with an even number of piston-cylinder units, in that their stress relieving corrugations can be constructed in pairs with one another. By the connection of the piston-cylinder units with the stress relieving corrugations, the pressure curve in the cylinders is smoothed out. Further, the coil spring valve elements between the two connected piston-cylinder units are already lifted, as soon as the first piston-cylinder unit reaches the opening pressure threshold, and remains open until pressure drops below the closure pressure of the second piston-cylinder unit.
In this way, not only the small thrust of the coil spring valve, but also the cutting in half of the number of the opening incidents, substantially reduces the noise.
A variant exists, in that the conduit is formed by a boring, which runs from one cylinder to a stress relieving zone of the neighboring piston-cylinder unit which lags behind in the direction of rotation. In this way, hydraulic fluid from the piston-cylinder unit which precedes in the transport thrust, is fed to the stress relieving zone, especially to the stress relieving corrugations of a piston-cylinder unit, which lags in the transport thrust.
In this way a smoothing of the curves in the cylinders which are bound together occurs, and the opening pressure of coil spring valve will be reached earlier, so that a more uniform transport exists and pressure variations of a high order are prevented. Radial piston pumps with eight piston-cylinder units and with stress relieving corrugations that are connected to one another in pairs, possess on this account still a pressure oscillation of the fourth power.
In order to improve the intake by suction of cold, high viscosity oil, it is of advantage, to place in each boring, a check valve. This so acts, that during the intake operations, the suction in the cylinder is improved, and in spite of it being during the transport thrust, the neighboring piston-cylinder unit is supplied with hydraulic fluid.
Instead of, or in addition to, the check valve, a temperature sensitive orifice can be provided in the boring, which reduces the cross section of the boring at low temperatures, or, in an extreme case, can close the opening. The orifice can also be designed as a control valve, which is controlled in relation to additional directive parameters from an electronic control unit.
Similar effects can be achieved by an annular suction groove, which connect the suction openings with each other. In this case, however, one has to tolerate overlapping of the control times of the currently connected cylinders, which, when taken as a whole, result in a somewhat reduced pump delivery. In order to compensate for this effect, the suction openings of the cylinder can be designed alternately larger and smaller about the circumference. By means of the connection of two cylinders, then the possibility exists, that during the suction thrust of a first cylinder having a larger suction opening, support is acquired from a greater suction from a second cylinder connected to it, whereby the first cylinder fills better, thus compensating for an otherwise occurring loss.